This invention relates generally to coil forms, and more particularly to coil forms having surfaces of high dimensional accuracy on which windings can be positioned with great accuracy.
In manufacturing certain types of electronic equipment such as radio frequency transmitters and receivers the need exists for inductance coils which can be economically manufactured in large quantities with consistent and predictable electrical characteristics. This is particularly true with coils which are self-resonant at RF frequencies, such as those used in antenna tuning applications, wherein the distributed capacitance of the coil is relied on to form a resonant circuit and no variable tuning means are provided.
While injection molding techniques allow forms for such coils to be produced at greatly reduced cost, the technique has been generally successful only for coils having a very small diameter; i.e., approximately 1/2 inch or less. For coil forms of larger sizes, e.g., from approximately 1/2 to 3 or more inches in diameter, the uneven shrinkage of the plastic following the molding operation has prevented the formation of a surface capable of supporting and positioning windings with the necessary precision to ensure a consistent and predictable self-resonant frequency. The problem is compounded in applications such as the aforementioned antenna tuning application wherein the coil form must, in addition to being dimensionally accurate, be capable of withstanding extreme temperature variations and repeated mechanical stresses.
It is therefore an object of the present invention to provide a new and improved injection-molded coil form having a high precision surface for supporting and positioning windings.
It is another object of the invention to provide a coil form having a precision surface for receiving windings which can be economically formed from plastic by injection-molding techniques.
It is another object of the invention to provide a high precision injected-molded coil form which offers improved resistance to temperature changes and mechanical stress.